Autumnal equinox

ASTRONOMY 161

Introduction to Solar System Astronomy

Seasons & Calendars

Monday, January 8

Season & Calendars: Key Concepts

(1) The cause of the seasons is the tilt of the Earth's rotation axis relative to its orbit around the Sun.

(2) The day is based on the time between one noon and the next.

(3) The year is based on the time between one vernal equinox and the next.

(4) The moon (month) is based on the time between one new moon and the next.

What causes the Earth's seasons?

NOT the varying distance of the Earth from the Sun.

Earth's orbit around the Sun:

1) The Earth's orbit is nearly circular: solar heating is nearly constant.

2) The Earth's closest approach to the Sun is in JANUARY.

3) Summer in Northern Hemisphere = Winter in Southern Hemisphere (and vice versa).

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